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We propose a novel method for fabricating fused fiber couplers whose transmission spectrum and sensitivity can be controlled in the wet-etching process. A wide tuning of the transmission spectrum during wet etching was observed experimentally. It was demonstrated that etched fused fiber couplers with reduced waist size had high sensitivity to the refractive index of an external medium. The sensitivity achieved by the present fiber coupler sensor was a 1125 nm/refractive index unit.